1. Field of the Invention
This invention relates to a light modulating apparatus, and more particularly to a light modulator in which a plurality of coaxially introduced light beams having different wavelengths are modulated by a single light modulator.
2. Description of the Prior Art
In a system for reading out image information on an original for example by scanning the original with a light beam such as laser beam and detecting the intensity of light reflected from the original, it is necessary to use a plurality of laser beams having different wavelengths, for example red (R), green (G) and blue (B), for the purpose of reading a color original. On the other hand, it is known to use sharp beams (for example beams having a diameter of 25 .mu.m) and unsharp beams (for example beams having a diameter of 125 .mu.m) as the reading laser beams in the read-out of image for the purpose of generating the edge effect on the reproduced image when the image is reproduced by on-off modulating the laser beams based on the image signal obtained by the read-out operation. Namely, six laser beams consisting of the R, G and B sharp beams and the R, G and B unsharp beams are used to read out the image. Usually, these laser beams reflected from the original are received by a single light receiver and converted to electric signals. Thereafter, the electric signals obtained from the R, G and B sharp beams must be separated from those obtained from the R, G and B unsharp beams.
In this case, it is advantageous to modulate either the sharp beams or the unsharp in advance and to separated the modulated sharp or unsharp beams from the unmodulated beams. Conventionally, three light modulators are generally used for the purpose of the aforesaid modulation, and the R, G and B unsharp beams are modulated for example at 4 MHz. However, the conventional modulating method is disadvantageous in that it requires an apparatus of complicated construction and is extremely expensive to conduct partially because three light modulators must be used as described above.
In the aforesaid modulation, the R, G and B unsharp beams are modulated in the same form, and usually coaxially superposed one upon another. Therefore, it is advantageous to modulate the R, G and B unsharp beams by use of a single light modulator. This is also effective to simplify the construction of the apparatus and to reduce the cost thereof.
However, when the R, G and B unsharp beams are coaxially introduced into the light modulator, the first-order diffracted lights of the R, G and B unsharp beams are emitted from the light modulator in different directions according to Bragg's condition.